The invention relates to a high-frequency bandpass filter, comprising several cylindrical conductor rods arranged in a line with predetermined spacings in a continuous space defined by an elongated housing made of an electrically conductive material and closed on all sides, each conductor rod being attached and short-circuited at its first end to the housing and spaced apart from the housing at its second end so that each conductor rod forms a coaxial resonator together with the housing.
In a typical high-frequency bandpass filter of the type described above, the conductor rods in the metal housing are separated from each other by partition walls into separate compartments each forming a coaxial resonator. The coupling between adjacent resonators is accomplished either by means of separate coil structures inductively at the short-circuited end of the resonators or by means of separate capacitor structures at the open end of the resonators. Another common practice is to realize each coaxial resonator by means of a conductor rod positioned in a fully separate metal box. The coupling between the resonators is again accomplished, e.g., by separate coil structures, such as a conductor wire running from one resonator box to another through coupling openings. Prior art filter structures of this type are large in size and complicated, in addition to which they require plenty of manual work and are difficult to tune, as a result of which a sufficiently accurate reproducibility of the desired filter properties is also difficult to achieve in series production. For instance, when using the above-mentioned conductor wire coil, the coupling between the resonators has to be adjusted by bending the conductor wire coil.
Another known filter type is the so-called Comb-Line filter, in which all conductor rods are placed, in place of separate metal boxes or compartments separated from each other by partition walls, in a single continuous space defined by a housing, so that an open filter structure is achieved, in which the couplings between the resonators are formed directly by the couplings between the conductor rods of the resonators. Therefore the filter is smaller in size and simpler than the filters described above. In this type of filter, the couplings between the conductor rods are controlled by means of adjustment screws provided in the cover of the housing and by varying the distances between the conductor rods; such adjustments, however, cannot provide different filter responses by one and the same filter unit for different applications.